1. Field of the Invention
The present invention relates to a method for the preparation of graphene, and particularly relates to a method for the preparation of a graphene dispersion containing a surface-modified graphene.
2. The Prior Arts
Graphene has been commonly referred to as monolayer graphite whose structure is one-atom-thick planar sheets of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. Graphene has mixed covalent and metallic bonding. In 2004, Andre Geim and Konstantin at the Manchester University obtained monolayer graphene by using an adhesive tape to repeatedly split graphite crystals. In 2010, the Nobel Prize in Physics was awarded to Andre Geim and Konstantin.
Graphene appears to be the thinnest and strongest materials ever tested. Thermal conductivity of graphene is higher than that of carbon nanotube and diamond. Electron mobility of graphene is higher than that of carbon nanotube and silicon crystal at room temperature. The resistivity of graphene is less than that of copper or silver, and graphene is the lowest resistivity substance ever known. Graphene and carbon nanotube have high flexibility and low optical reflectivity so that they are used as a preferred soft electronic material. However, the graphene dispersion is more difficult to be coated in comparison with the carbon nanotube dispersion. Graphene tends to aggregate irreversibly into large clusters and even restacks to form graphite. New strategies to produce a uniform monolayer graphene film while keeping them individually separated are required.
Graphene can be produced by exfoliation, direct growth, or produced from carbon nanotubes, wherein graphene powder can be produced by using exfoliation techniques. However, graphene can be mass-produced via the oxidation and reduction process, and in such a process, a graphite material is oxidized to form graphite oxide, then graphene is obtained by the exfoliation and reduction of graphite oxide.
In US 20100237296, graphene oxide is dispersed into water to form a dispersion, and then a water-insoluble organic solvent is added to the dispersion, followed by heating at about 200° C. under stirring to form graphene. Although the organic solvent is used as reducing agent in US 20100237296, its reductibility is not high enough. Therefore, the operation temperature has to be increased to as high as the boiling point of the organic solvent so that the organic solvent is required to be continuously added during the reduction process.
In U.S. Pat. No. 7,824,651, graphite is directly placed in a solution containing a dispersing agent to obtain a suspension, and the suspension is exposed to ultrasonic waves at an energy level of greater than 80 watts to exfoliate graphite into a monolayer graphene with a thickness smaller than 10 nm. However, it takes a long time to obtain the desired size of graphene by mechanical force, which causes the increase of the energy consumption.
In U.S. Pat. No. 7,658,901, the thermally exfoliated graphite oxide in powder form is obtained from heating graphite oxide, and the thermally exfoliated graphite oxide is placed over a hot source under protective atmosphere for a period of time to obtain monolayer graphene. This method is simple, but the powder size and the oxygen content in each batch are hard to be controlled, and thereby the stability of the product is low.
In US 20100303706, the graphite oxide is placed into a basic solution containing hydrazine or NaBH4 under stirring, and consequently graphene oxide is reduced to form a graphene dispersion by hydrazine or NaBH4. However, hydrazine is highly toxic and usage should be minimized. If graphite oxide is reduced in a non-basic solution, the degree of agglomeration in the obtained graphene dispersion becomes serious.
Songfeng Pei et al. in Carbon (journal) of the year 2010 disclosed that a graphene oxide film can be reduced into a graphene film by HI acidic solution. They also found that if the graphene oxide film is reduced into a graphene film by hydrazine, the graphene film will break up in water to form a dispersion. However, the graphene film is kept in an acidic solution due to agglomeration.
Accordingly, the reduction of graphene oxide suspension should be carried out in a basic environment in order to form a graphene dispersion with uniform dispersibility. If the reduction of graphene oxide suspension is carried out in a non-basic environment, graphene tends to agglomerate or even restack upon reduction, so that the formed graphene dispersion contains agglomerates.